The invention relates to a method for producing fine microcrystalline titanium dioxide in the rutile form, with a crystal size of less than 15 nm.
Typically, microcrystalline titanium dioxide refers to titanium dioxide, TiO2, with crystal sizes 5 to 10 times smaller than those of pigmentary titanium dioxide. Typically, the crystal size of microcrystalline titanium dioxide is of the order of 10 to 100 nm, resulting in optical properties of high transmission of visible light. On the other hand, UV radiation is well filtered i.e. absorbed, and reflected by transparent titanium dioxide. Accordingly, microcrystalline titanium dioxide is used in various applications such as in sunscreens where UV protection and superior transparency are required. For these applications, the best results are attained with microcrystalline titanium dioxide in the rutile form. An advantage of the titanium dioxide crystals in the rutile form is the wider UV protection provided in comparison with the anatase form.
Microcrystalline titanium dioxide is produced by methods associated with particular challenges to attain a controlled small crystal size, narrow crystal size distribution, and a suitable crystalline form, and further to prevent the liability of the particles to agglomerate, and to improve the dispersibility thereof. Moreover, requirements for the production processes are typically economic efficiency, and use of ecologically acceptable materials and process steps.
There are various methods for producing microcrystalline titanium dioxide, such as gas phase methods and methods starting from aqueous precipitates. Typically in the gas phase methods, anatase is first formed in the crystallization process of titanium dioxide, since it is a thermodynamically more stable structural form in the initial stages of the crystal growth. This often results in greater specific surface area of titanium dioxide in the anatase form due to smaller crystals formed. This is especially true for in the gas phase reaction with a high reaction rate, that is, the shorter reaction time is not sufficient for actual crystal growth of the rutile form. With this method, crystals in the rutile form may be produced by elevating the reaction temperature sufficiently, the higher temperature making the rutile form thermodynamically more favourable. Crystal sizes being, however, strongly increased by such high reaction temperatures, sizes of 10 nm may no longer be attained.
U.S. Pat. No. 5,698,177 discloses a gas phase synthesis for the production of TiO2 where dopants compatible with TiCl4 to be volatilized are used for the modification of the physical properties of TiO2. SiCl4 is mentioned as a possible dopant enabling to maximize the amount of the anatase form and to minimize the particle size thereof, thus increasing the specific surface area of the final product. However, handling of TiCl4 is extremely difficult. It is very reactive and liable to form chloro compounds in the gas phase. Moreover, purity requirements for the gas phase reaction are high since all impurities will enter the titanium dioxide crystal in the burning step. Control of the anatase phase and specific surface area thereof is presented in the document where the proportion of the rutile form remains below 17%.
EP Patent No. 1514846 discloses a gas phase synthesis carried out with plasma, using a SiO2 precursor mainly for the prevention of agglomeration, thus reducing the amount of particles with excessive sizes and increasing the specific surface area.
A previous patent of the applicant, EP Patent No. 0444798, describes an aqueous process for the production of microcrystalline titanium oxide in the rutile form, having a crystal size of less than 100 nm. In the process, solid titanium dioxide hydrate is first treated with a base, preferably with aqueous sodium hydroxide to reach an alkaline pH. Thereafter, the precipitate treated with the base is acidified with hydrochloric acid having a final concentration adjusted between 8 and 25 g/l. The precipitate obtained by the base and acid treatments is neutralized, preferably to a pH of 4 to 6. With this process, titanium dioxide with the proportion of the rutile form of 99.5%, and a crystal size typically of the order of 25 nm is obtained, crystal sizes of even as low as 15 nm being obtained by suitable selection of the process parameters.
U.S. Pat. No. 5,536,448 discloses the production of dendrite or asteroidal TiO2 by a liquid phase process. Base is added to an aqueous dispersion of titanium dioxide hydrate to adjust pH to a neutral value, followed by heating of the mixture to a temperature between 90 and 100° C. The product obtained is further dispersed in water and mixed with hydrochloric acid in a molar ratio of HCl to TiO2 of 1-4:1. The dispersion is aged at 85 to 100° C. for several hours, giving rise to dendrite or asteroidal TiO2 microparticles. These product particles may be coated for instance with silicon oxides or oxyhydrates by slurrying the titanium dioxide product in water and adding a water-soluble silicon salt to the solution thus obtained. The coating deposited on the surfaces of the titanium dioxide particles improves the dispersibility of the product, and the stability of the dispersion.
Microcrystalline titanium oxide may also be produced by thermal hydrolysis of titanium oxochloride as described for instance in Bekkerman, L. I.; Dobrovol'skii, I. P.; Ivakin, A. A. Effects of the composition of titanium(IV) solutions and precipitation conditions on the structure of the solid phase. Russ. J. Inorg. Chem. 1976, 21, 223-226, or by neutralization, e.g. by heating a mixture of titanium oxychloride and a solvent to 60° C., addition of NaOH and calcinating the product thus obtained, as disclosed in EP Patent No. 1443023. The drawback of these processes is typically the fact that the starting material for the oxychloride solution, titanium tetrachloride, is difficult to handle due to liability to vaporize, and further, due to necessary cooling during dilution. Moreover, the use of the method of this type is typically associated with corrosion problems, safety risks and relatively high production costs.
The object of the present invention is to provide microcrystalline titanium dioxide in the rutile form, having superior transmission of visual light and ability to inhibit UV radiation. Another object is to produce microcrystalline titanium dioxide in the rutile form by a safe and industrially feasible method.